250 C. p. RHOADS 



factor are so numerous and the results so well defined that reference to 

 the original material is required for an adequate consideration of the 

 subject. 



The reports of Green (117) have been particularly cogent in invok- 

 ing studies of the milk agent in support of a virus theory of cancer. The 

 theory is based clearly upon a concept of the disease as due to a "parasite 

 entirely foreign to normal mouse tissue. The origin of viruses from 

 microbes and their nature as highly adapted parasites are reasonable 

 concepts and correspond to the course of parasitism and the nature of 

 highly adapted parasites as observed throughout the biological world." 



The milk agent is reported as having the following characteristics: 

 It is present in the milk secreted throughout the lactation period, and in 

 blood and normal tissue as well as in the spontaneous mammary cancer, 

 originally or after serial passages by nursing during the precancerous 

 stage. When passed serially by injection through highly susceptible but 

 not infected mice, the power to cause tumor declined and was lost by 

 the third passage (118). Dilute suspensions are more active than con- 

 centrated ones (119). The agent is as active by injection as when given 

 by mouth and resists drying, filtration, lyophilization, and treatment 

 with glycerine. It is heat labile and destroyed by extremes of pli. Most 

 of the activity is sedimented by centrifugation at 40,000 r.p.m. (iio,- 

 000 g. ) for one hour. These characteristics, like the ones shown for the 

 antigen in the Brown-Pearce tumor, are the ones of a large protein 

 molecule. 



The studies of Andervont and Bryan (120) and of Green (12) show 

 conclusively that the milk agent is active as an antigen when centrifu- 

 gates of tumor tissue, collected between runs at 15,000 g. and 95,000 g., 

 are injected into rats and rabbits. None of the animals given a mixture 

 of immune serum and agent developed tumors, whereas a large propor- 

 tion of the controls which received mixtures of the agent with normal 

 mouse tissue antiserum, normal rat serum, or saline did so. These re- 

 sults are interpreted as evidence that "the agent is a virus of exogenous 



origin." 



An equally striking experiment has been reported by Green (13) on 

 the toxic properties of the mouse milk agent antiserum for the mouse 

 mammary cancer cells. Samples of a suspension of mouse mammary 

 cancer cells were mixed with the agent antiserum, normal breast tissue 

 antiserum, normal rabbit serum and saline solution, and injected into 

 susceptible mice. None of the animals which received the mixtures of 



